Various commercial processes are available in the prior art for making glass sheet materials. Float process, as an example, involves the formation of glass sheet on a molten metal bath. This process can be used for making glass sheet material with a wide range of compositions. Another advantage of this process is the capability of achieving high surface quality on the side exposed to air. However, this process requires direct contact between the metal melt and the glass melt, which can lead to unwanted contamination of the interface and less than pristine surface quality. Thus, in order to produce high quality glass sheet with pristine surface properties on both major surfaces, glass sheet produced from the float process may need to be further surface-polished, which can be a quite expensive step. Moreover, it is believed that the float process has not been used for making thin, rollable glass ribbons.
Fusion draw process is one of the current processes for making thin glass sheet with extremely high surface quality, such as those for use as LCD substrates. Because both major surfaces of the glass sheet are not allowed to directly contact any support material during the forming process, high surface quality of both of them can be achieved simultaneously. However, to be eligible for this process, the glass must meet certain requirements regarding liquidus viscosity, strain point, and the like. Therefore, this process cannot be used efficiently and effectively for many glass compositions. Moreover, it is known that this process requires the use of expensive machinery. Even though the LCD glass substrates made by fusion draw processes are quite thin, they are not thin enough to have the flexibility needed or desired for many applications.
Particularly, for high-silica glass sheet, such as glass sheet materials comprising at least 90% by mole, float process and fusion draw are impractical due to the high softening point of the glass material. Large-size high-purity silica glass sheets are useful, for example, as regular photomask substrates, LCD imagemask substrates, and the like. Such silica glass substrates are typically produced by cutting, grinding and polishing of large silica glass ingots produced, e.g., in large flame-hydrolysis furnaces, an extremely expensive and wasteful process. Such slicing and polishing approaches would be prohibitively expensive for making thin, flexible silica glass sheet with a thickness lower than 500 μm, good thickness uniformity and decent surface quality. A less expensive alternative process for making silica glass sheet, desirably with high surface quality and thickness uniformity, would be highly desirable.
By providing a soot process for making glass sheet materials, certain embodiments of the present invention satisfies the above need.